This is a common myth but is wrong. Smaller boxes will increase the response in the upper bass frequencies and start to become peaky or boomy. As a sealed box gets smaller the Qtc value rises, which hurts its low frequency response and also increases the transient response of the speaker. With a bigger box the Qtc will get lower and the transient response and low frequency response will improve. Look at the following link to see how frequency response changes with Qtc.
http://www.loudspeakers101.com/SealHelp.htm For the best sound you want the frequency response to be as flat as possible, which means about a Qtc of .707 for a woofer than plays up to several hundred hertz. For a subwoofer that will only play up to 75 Hz or so, you get the flattest response by having as low a Qtc as possible. And remember to lower Qtc you increase the size of the box and to maximize it you mount the sub infinite baffle.
Some people may prefer the sound of a small box because it plays midbass frequencies louder and their other speakers don't play these will. But a subwoofer should only play up to about 75 Hz because higher than this the sound starts to get directional, and to sound best these frequencies should only be played with the front speakers. Also since a car has a gain at lower frequencies because of the small interior, the loss in response by the subwoofer may be made up by, by the cabin gain.